The present invention relates to a flexible printed circuit board and an integrated circuit chip mounting flexible printed circuit board that have suitable properties for being mounted on a display panel, such as a liquid crystal display panel, and to a display apparatus incorporating same. The present invention also relates to an integrated circuit chip mounted structure and a bonding method of the integrated circuit chip mounting flexible printed circuit board. Especially, the present invention relates to a flexible printed circuit board and an integrated circuit chip mounting flexible printed circuit board that are provided with a wiring structure characteristic in its length and width, and an excellent bonding reliability with respect to an integrated circuit chip, and further relates to a display apparatus incorporating same.
An example of a conventional method of mounting a integrated circuit (IC) chip on a flexible printed circuit (FPC) board, which is to be mounted on a liquid crystal display panel, is a inner-lead bonding method that makes good use of an Auxe2x80x94Sn eutectic reaction.
In general, the bonding method is employed to bond an IC chip having bumps of Au (gold), and an FPC board having three layers, namely a PI (polyimid) film as a board main body, an adhesive agent layer, and copper-foiled wires with a Sn (tin)-plated surface, which is fixed on the PI film by the adhesive agent layer. The copper-foiled wires and the bumps are treated by thermocompression bonding so as to create a molten Auxe2x80x94Sn alloy in a bonding interface between the copper-foiled wires and the bumps. As a result, the three-layered FPC board and the IC chip are electrically connected. Moreover, the joint parts are reinforced and protected by applying a resin around the IC chip after the IC chip is mounted.
However, to cope with recent demand for a greater output, the pitches between the bumps of the IC chip has been rapidly reduced so as to provide an IC chip with fine-pitched bumps. As an FPC board for such an IC chip, employed is a two-layered FPC board, which is an FPC board having a similar structure as the conventional FPC board, that is the three-layered FPC board, but having no adhesive agent layer. In the two-layered FPC board, the copper-foiled wires are directly provided on the PI film as the board main body by applying a plating or an etching method. Compared with the three-layered FPC board, the two-layered FPC board has an advantage that the copper-foiled wires are accurately positioned with fine pitches, while such a problem is posed that adhesion of the copper-foiled wires onto the PI film is weak.
When those FPC board and IC chip with fine pitches are bonded, it is not rare to have undesirable problems, such as, inadequate electrical connection reliability due to insufficient thermocompression bonding. In other words, without a concrete rule in terms of designing and production of the IC chip mounting FPC board, the wires of the IC chip and the FPC board cannot be electrically connected to each other sufficiently. This may causes an open phenomenon or a leakage, for example, because of misregistration between the joint parts of the IC chip and that of the FPC board, in contact. Thus, it is very important for attaining good bonding reliability that pressures (load per unit area) are evenly applied onto each copper-foiled wire during the thermocompression bonding, and the misregistration and twisting of the bonded two layers are prevented, when the two layers are bonded each other. Those points are especially important when the bonding is carried out by using an anisotropic conductive material, which is prepared by defusing conductive particles in a thermohardening resin. A typical example of the thermohardening resin is an epoxy resin, while the conductive particles may be resin particles treated by plating or metal particles, for example.
As an example of a terminal structure (a wiring structure) of the IC chip mounting FPC board, presented here is a terminal structure having a mounting region in a rectangular shape that corresponds to a shape of the IC chip to be mounted. Such a terminal structure is provided with the copper-foiled wires (inner-leads) for bonding in the mounting region. The inner-leads are disposed on facing two sides of the mounting resin, while numbers of the inner-leads may be same or different between the facing two sides, while the inner-leads are identical in terms of a width.
In such a terminal structure having the inner-leads that have an identical width and are disposed in the same number on each of facing two sides of the mounting region for the IC chip, the pressure on each inner-lead is applied substantially equal between the respective facing two sides of the mounting region during the thermocompression bonding, as long as all the inner-leads are respectively connected with the associating bumps (that is, bumps to be connected with the inner-leads) that are disposed on the IC chip. Especially, it is possible to attain good connection when the anisotropic conductive material is used. However, for example, in case some of the inner-leads are not connected with the bumps and left over, the respective inner-leads may be unevenly pressured during the thermocompression bonding due to a difference in total areas of superimposing parts of the inner-leads for overlapping with the bumps (that is, the bump-associating contacting parts locating on bonding parts of the inner-leads) (hereinafter, just refer to as an superimposing region) between the facing two sides of the mounting region. This may result in inadequate bonding. Moreover, with a structure having different numbers of the inner-leads respectively on the facing two sides of the mounting region, the two sides may be similarly pressured unevenly during the thermocompression bonding, because of the difference in the total areas of superimposing regions of the inner-leads between the facing two sides.
In short, to uniform the width of the inner-leads and the wire pitches is not sufficient enough to attain the even pressure applied on each bonding interface between the bump on the IC chip and the copper-foiled wires on the FPC board. Hence, in order to solve the problem of unbalance pressure during the thermocompression bonding, it is necessary to have, for example, a structure in which the IC chip is mounted on an FPC board provided with wiring that is able to absorb an effect of the unbalance between the pressures applied onto the respective joint parts of the facing two sides due to the difference between the numbers of output terminals (the bumps) provided on the facing two sides of the IC chip.
In order to carry out a mass production while ensuring realization of a mounting structure with the consideration of the pressure balance between the facing two sides of the mounting region for the IC chip, it is required to design the copper-foiled wires on the FPC board in view of, for example, positioning accuracy for the IC chip mounting. For instance, a length of the inner-leads can be designed to be extended toward the circuit with respect to positions of the bumps, in order to absorb an effect of the misregistration. However, the extension should not be longer than a certain upper limit, lest that it cause drawbacks such as leakage due to terminals twisted during the thermocompression bonding of the IC chip. Moreover, the width of the inner-leads need be designed to be thinner than the width of the bumps, so that no leakage is caused between the terminals even when the misregistration happens.
Furthermore, in case there are a large difference in the numbers of the inner-leads between the facing two sides of the mounting region for the IC chip, the above designs may not be sufficient enough to substantially equalize the total areas of the superimposing regions between the facing two sides. This case may be countered by setting a minimal requirement for a ratio between the total areas of the superimposing regions of the facing two sides. Satisfying the minimal requirement for the ratio will achieve a regular supply of an IC chip mounting FPC board provided with a stable quality.
The present invention has an object of offering an FPC board and an IC chip mounting FPC board with a design to balance a bonding pressure onto each joint part and with excellent bonding reliability. With the design, each joint part is evenly pressured even when facing two sides of an IC chip in a substantially quadrangular shape have different numbers of output terminals (bumps), in other words, when difference numbers of inner-leads are respectively provided on facing two sides of a mounting region in which the IC chip is mounted on the FPC board. Another object of the present invention is to offer a display apparatus incorporating same, while its still another object is to offer a bonding method of the IC chip mounting FPC board. Moreover, yet another object of the present invention is to offer an IC chip mounted structure in which an IC chip is mounted on the FPC board.
In order to achieve the objects, the IC chip mounting FPC board of the present invention is provided with (1) a mounting region in a substantially quadrangular shape for mounting an integrated circuit chip, (2) a plurality of inner-leads, provided along each side of the mounting region, for being connected with bumps of the integrated circuit chip so that the integrated circuit chip is mounted and bonded in the mounting region, wherein the inner-leads are provided so as to have substantial equality between (a) a sum of areas of superimposing regions for the inner-leads provided on one of facing two sides and the bumps, and (b) a sum of areas of superimposing regions for the inner-leads provided on the other of the facing two sides and the bumps.
With the arrangement, achieved is substantial equality between (a) the sum of the areas of the superimposing regions of the inner-leads provided on one of facing two sides, and (b) the sum of the areas of the superimposing regions of the inner-leads provided on the other of the facing two sides. This equality leads to substantial evenness in the pressure applied on the inner-leads when the IC chip is mounted on the mounting region. This prevents the bonding pressure during the IC chip mounting from being unbalanced. Thus, it is possible to realize the IC chip mounting FPC board with the high bonding reliability.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.